Influence of laser power density on the mesostructure of thermoplastic composite preforms manufactured by Automated Fibre Placement

Automated Fibre Placement is an established composite manufacturing technology for the deposition of carbon fibre reinforced thermoset materials. In recent years, implementation of suitably controlled laser heaters has enabled processing of high-performance thermoplastic matrix prepreg at a rate compatible with industrial environments. Laser-assisted Automated Fibre Placement is often characterised solely in terms of the supplied laser power or the measured temperature at the convergence between the layup roller and the substrate, referred to as the "nippoint". However, the thermally affected zone extends beyond the nip-point, and the thermal profile in its vicinity determines the meso-structure of the deposited thermoplastic composite laminate. Specifically, the void fraction and degree of inter-ply bonding. 

An infrared thermal camera and embedded thermocouples were used to determine the thermal profile resulting from the use of two laser lenses, delivering different laser beam cross-sectional depths. The resulting laminate structure and void content were assessed by optical microscopy of polished cross-sections and ultrasonic testing. 

This work demonstrates that, for constant nip point temperature, the size of the laser heated zone relative to the nip-point alters the resulting void fraction and inter-ply bonding.